This invention relates generally to nozzle for gas turbine engines and more particularly to a variable geometry convergent-divergent nozzle.
Exhaust systems for gas turbine engines which operate over a wide range of pressure ratios (i.e. nozzle throat pressure/ambient pressure or “P8/Pamb”) require variable geometry in order to adjust the nozzle throat area (“A8”) to meet the demands of the engine cycle, and adjust the nozzle expansion ratio (i.e. nozzle exit area/nozzle throat area or “A9/A8”) in order to attain good performance at the various operating points.
Prior art exhaust nozzles include fixed exhaust systems typical of commercial subsonic engines, and variable exhaust nozzles typical of supersonic military aircraft which also use afterburners. Fixed nozzles do not kinematically change their geometry and thus are not designed to operate efficiently over a wide range of nozzle pressure ratios (P8/Pamb).
In prior art variable geometry exhaust nozzles, throat area A8 and expansion ratio control has typically been established by “linking” the A9/A8 ratio to A8 (a kinematically linked area ratio schedule). For example, a circumferential series of overlapping flaps and seals may be used to create a convergent flowpath that establishes A8. A similar set of overlapping flaps and seals is connected to the aft end of the convergent flaps and seals and establishes the divergent portion of the nozzle and thus defines the exit area A9 of the nozzle. The divergent flaps are also kinematically linked via a separate kinematic member (a compression link for instance) to a relatively stationary part of the engine exhaust system such as a duct. The resulting four-bar linkage (duct, convergent flap, divergent flap, compression link) defines the kinematic relationship of the exit area, A9, to the nozzle throat area, A8; and thus defines the A9/A8 ratio schedule as a function of A8. This typically results in an A9/A8 schedule which increases as A8 increases. This type of nozzle design has several disadvantages. Because of the overlapping flap & seal structure, there are numerous leakage paths which reduce operating efficiency, and the large number of parts required increases cost, weight, and maintenance effort, and decreases reliability. Furthermore, for a number of engine cycles, the scheduled A9/A8 ratio vs. A8 relationship will not match the engine cycle demands optimally and thus will not deliver peak nozzle performance at certain key operating points.
Although prior art overlapping flap and seal nozzles exist which enable independent A9 and A8 control they still suffer from excessive complexity and sealing difficulties.
Exhaust systems have been proposed with translating contoured shrouds and fixed internal plugs which would enable some A8 variation. This results in a “scheduled” A9/A8 characteristic where for each A8 there is a unique A9/A8. The translating shroud design is much simpler than the overlapping flap and seal nozzle, has fewer leakage paths, and can be substantially lighter; however, if the engine cycle demands two vastly different nozzle pressure ratios at a given nozzle throat area A8 (for example: P8/Pamb=2.5 at one condition and P8/Pamb=20.0 at another flight condition with nearly the same A8), the nozzle will not be able to attain a geometry which results in good performance for both points.
Accordingly, there is a need for an exhaust nozzle which provides independent control of the throat area and the expansion ratio using a simple, robust structure.